1. Field of the Invention
This invention relates to a crankshaft mechanism having a variable displacement or stroke. In particular, it relates to a crankshaft mechanism for driving a slide member of a press, wherein the mechanism can be adjusted, such as for a minimum or a maximum stroke.
2. Description of the Related Art
The stroke of a crankshaft, and therefore the displacement of the slide member, is defined herein as the distance the crankshaft moves between its top dead center and bottom dead center positions, unless otherwise specified.
The pressing rate (N) and the slide stroke (S) of a press have an interrelationship such as that shown in FIG. 8. Such an interrelationship, determined by the structure of the particular press, is defined by an inherent characteristic thereof, namely its ratio of slide stroke to pressing rate (S/N). Accordingly, if the S/N ratio is adjusted, it is possible to selectively operate a single press at various rates and levels of precision.
For this purpose, adjustable crankshaft mechanisms have been developed, and such a mechanism is schematically illustrated in FIG. 9. Referring to FIG. 9, a cylindrical crankshaft 10, with a longitudinal axis of rotation (crankshaft axis) Z1, includes an enlarged eccentric portion 11 having a center axis Z2 spaced radially a distance (hereinafter "eccentricity") e1 from the axis Z1. An eccentric sheave 20 is fitted on the outer circumference of the eccentric portion 11, and has a center axis (sheath axis) Z3 spaced radially a distance (eccentricity) e2 from the axis Z2 of the eccentric portion 11. Thus, a multieccentric structure is formed. A connecting rod 3 has an upper end portion 3U, rotatably mounted on the circumference of the eccentric sheave 20, and a lower end portion swingably supporting a slide 1. The angular position of the eccentric sheave 20 with respect to the eccentric portion 11 is adjustable. Therefore, by adjusting the relative angular positions of the eccentric portion 11 and the eccentric sheave 20, it is possible to freely change the composite eccentricity with respect to the crankshaft axis Z1 (distance of the sheath axis Z3 from the crankshaft axis Z1), so as to adjust the stroke of the slide 1. In particular, the composite eccentricity can be set at values within the range from le1-e2l to (e1+e2). The minimum composite eccentricity le1-e2l corresponds to when the eccentricities e1 and e2 are in opposite directions, and thus opposite to that shown in FIG. 9. The maximum composite eccentricity (e1+e2) corresponds to when the eccentricities e1 and e2 are in the same direction, as illustrated in FIG. 9.
Mechanisms for adjusting the slide stroke are known, for example, from Japanese Patent Publication No. 48-4356, Japanese Patent Publication No. 51-12150, Japanese Utility Model Publication No. 55-13039, U.S. Pat. No. 4,033,252 and German Patent No. 3112382. Such conventional mechanisms have a complicated structure, and the component parts have large diameters or other large dimensions, irrespective of the slide stroke. Furthermore, when the slide stroke is adjusted, the relationship between the rotational angle of the crankshaft (i.e., the crank angle), and the motion of the slide, is changed. This is disadvantageous in a press which is used in association with various different equipment. Also disadvantageous is that when the slide stroke is changed, the die height changes as well.
A crankshaft mechanism which has built-in (internal) means for switching between two different strokes, is known from U.S. Pat. No. 4,846,014. That crankshaft mechanism has parts with small diameters and other small dimensions. That mechanism also is capable of maintaining the same die height and the same relationship of crank angle to slide motion, even when the operative stroke is changed. A slide stroke adjusting apparatus having an external switching mechanism is known from Japanese Patent Laid-Open No. 4-105800.
Recently, demands have strengthened for presses for which capital and maintenance costs are reduced, working precision is increased, and the size in three-dimensions is decreased. However, even with the above-described crankshaft mechanisms, these demands sometimes cannot be met.
For example, an apparatus having a built-in switching mechanism, such as is disclosed in U.S. Pat. No. 4,846,014, includes a cylinder device having upper and lower pistons fitted in bores which are formed in the eccentric portion, the eccentric sheave, and the connecting rod. The bores can be aligned so that the pistons can fix the sheave alternatively to either the eccentric portion or the connection rod. This leaves the non-fixed one of the sheave and the eccentric portion free to rotate with respect the other. Such a construction requires a large number of component parts, and is thus expensive. In addition, this construction requires complicated and very precise machining and assembly. Further, replacement of the pistons, packings, etc. is a laborious operation.
With an apparatus having an external switching mechanism, it is necessary to provide a space within which the eccentric bush (eccentric sheave) can move in the axial direction of the crankshaft. Reductions in size therefore cannot easily be achieved in the narrow interior of the crown in which the crankshaft is mounted. Further, a rotational gap in the crankshaft bearing makes it essential to provide a gear backlash. Therefore, demands for higher precision cannot easily be met.